Impact of head immobilization position on dose distribution in patients of brainstem glioma

Abstract

Purpose: The purpose of this study is to investigate the impact of patient position (supine and prone) on conventional bilateral field, three dimensional conformal radiotherapy (3DCRT) and intensity modulated radiotherapy (IMRT) treatment plans in patients of brainstem glioma with a view to exploring the possibility of avoiding beam entry through immobilization accessories. Methods: Five patients of brainstem glioma were immobilized and scanned in supine and prone positions with a combination of head rest and thermoplastic cast. Each patient was planned with three techniques: (i) 2-fields bilateral (ii) 3-fields 3DCRT, and (iii) 5-fields IMRT. Plan quality was analyzed in terms of planning target volume (PTV) coverage and dose to various critical organs at risk (OAR) for both the supine and prone treatment positions. Results: In case of bilateral fields (parallel opposed) planning, the PTV coverage and dose to the OAR were almost similar for both the supine and prone positions. In 3DCRT plan, although the PTV coverage and dose to critical structures were comparable for both the supine and prone position, dose to cochlea was lower for the prone position plan. A modest decrease in maximum dose to optic nerves and mean dose to temporal lobes were also observed for the prone position plan. In IMRT plans, the PTV coverage and homogeneity were comparable in both the supine and prone positions. Reduction in average maximum and mean doses to all OARs with functional subunit (FSU) in series and parallel respectively was observed in the IMRT plan for prone position when compared to the supine position. Conclusion: Supine and prone positions resulted in almost similar dose distribution in all the three techniques applied. At some instances, the prone position showed better normal tissues sparing when compared to supine. Moreover, prone position is more likely to avoid attenuation due to immobilization devices and uncertainty in dose calculation under large inhomogeneities.